1. Field of the Invention
The present invention relates generally to a device for determining voltage, electric current, temperature, and the like as physical quantities of a secondary battery to be mounted in a motor-driven vehicle such as an electric automobile or a hybrid vehicle, an uninterruptible power supply, a backup power supply, or the like. More specifically, this invention relates to a technique for accurately determining the physical quantities with respect to variations in environmental temperature of the device.
2. Related Background Art
Secondary batteries include a lead battery, a nickel-cadmium (Nixe2x80x94Cd) battery, a nickel-metal hydride (Nixe2x80x94MH) battery, a lithium-ion battery, and the like. These batteries have the following property. When the battery becomes exhausted, recharging can be performed by connecting the battery to an external power source so that a predetermined electric current is fed to the battery. Conventionally, this property has allowed these batteries to be used in various equipment.
For example, the battery is mounted in a vehicle to serve as a battery for supplying a spark plug of an engine with power when starting the engine. Recently, the Nixe2x80x94MH battery has been used also as a main power source for driving a motor in an electric automobile and a so-called hybrid vehicle that is provided with an engine and a motor. In this case, the Nixe2x80x94MH battery is used in the form of a battery pack as a combined body of a plurality of single secondary battery cells.
In the hybrid vehicle, the following are performed depending on an output level of the engine. When the output level of the engine is higher than a power level required for driving the vehicle, excess power is used to drive a generator so that the secondary battery is charged. On the contrary, when the output level of the engine is lower than the power level required for driving the vehicle, electric power from the secondary battery is used to drive the motor so that power to compensate for the power shortage is output. In this case, the secondary battery is discharged. It is necessary for the charging/discharging or the like to be controlled so that a proper operating state is maintained when a secondary battery is mounted in a hybrid vehicle or the like.
In order to meet the aforementioned requirement, output voltage, charge/discharge current, internal temperature, and the like of a battery pack are detected, and based on a result of the detection, a state of charge (SOC) of a battery is estimated. Based on a result of the estimation, the SOC is controlled so that the highest fuel consumption efficiency of a vehicle can be obtained. Accordingly, in order for the SOC of the battery to be controlled with reliability, it has been required to determine a physical quantity of the battery pack such as the output voltage, the charge/discharge current, the internal temperature, and the like with accuracy, regardless of variations in environmental temperature.
Conventionally, as shown in FIG. 3, a physical quantity of a battery pack has been determined in the following manner. Signals VB, IB, and TB from sensors (not shown) for detecting output voltage, charge/discharge current, and internal temperature of a battery pack 1 are received by an electronic controlling unit 30 (ECU). The respective signals VB, IB, and TB as analog signals are converted into digital signals in chronological order by an A/D converter 31 provided in the ECU 30. The respective physical quantities are determined by a microcomputer 32 or the like that is supplied with a supply voltage VO by a voltage regulator 33. Based on a result of the determination, an SOC of the battery pack 1 is estimated. A reference voltage VR for conversion performed in the A/D converter 31 is supplied by a reference voltage generating IC 34 that has a very low temperature coefficient (for example, a variation rate of a reference voltage is not more than 0.1% at temperatures ranging from xe2x88x9230xc2x0 C. to +80xc2x0 C.) and achieves high accuracy and high stability, so that the physical quantities are determined accurately with respect to variations in environmental temperature.
Furthermore, a technique has been proposed, in which as shown in FIG. 3, an environmental temperature TE is determined, and based on the result of the determination, temperature characteristics of a circuit system from the sensors for detecting the respective physical quantities to the A/D converter 31 in the ECU 30 are corrected (for example, JP2000-146620 A).
As has always been the case, the reference voltage generating IC 34 for generating a reference voltage with respect to the A/D converter 31 is used so that data as a result of conversion in the A/D converter 31 can be obtained with accuracy. The reference voltage generating IC provides the advantages of high accuracy and high stability. However, in obtaining the advantages, the reference voltage generating IC has been a cause of a substantial cost increase.
With the foregoing in mind, it is an object of the present invention to provide a device that allows a physical quantity of a battery pack to be determined at low cost with high accuracy with respect to variations in environmental temperature.
In order to achieve the aforementioned object, a device for determining a physical quantity of a battery pack of the present invention is a device for determining a physical quantity of a battery pack as a combined body of a plurality of single secondary battery cells. The device includes a physical quantity detecting part for generating a signal of a level corresponding to a value of a physical quantity of the battery pack to be detected, a temperature detecting part for generating a signal of a level corresponding to an ambient temperature in the device, and an A/D converter for converting analog signals from the physical quantity detecting part and the temperature detecting part into digital signals. The device further includes a storage part for storing in advance signal levels that are obtained from the physical quantity detecting part and the temperature detecting part via the A/D converter so as to correspond to a plurality of temperatures in a predetermined temperature range, a signal processing part for correcting a signal level obtained from the physical quantity detecting part via the A/D converter, based on the signal levels corresponding to the plurality of temperatures that have been stored in the storage part, and a voltage regulator for regulating a voltage level of an input power source to supply the temperature detecting part and the A/D converter with reference voltages as a reference voltage for detection and a reference voltage for conversion, respectively, and supply the storage part and the signal processing part with the reference voltage as a supply voltage. In this case, the secondary batteries are connected in series.
In this configuration, generally, the supply voltage that is supplied to the storage part and the signal processing part by the voltage regulator (such as a voltage regulator IC for general purpose use) varies within a range of voltage values that allow the storage part and the signal processing part to operate properly, depending on a temperature in the predetermined temperature range. The supply voltage that varies with varying environmental temperatures also is supplied to the temperature detecting part and the A/D converter as the reference voltage for detection and the reference voltage for conversion, respectively. The signal levels from the physical quantity detecting part and the temperature detecting part are stored in advance in the storage part via the A/D converter so as to correspond to the plurality of temperatures in the predetermined temperature range. In an actual determination of the physical quantity, based on the signal levels that are stored in advance in the storage part, a signal level from the physical quantity detecting part is corrected in the signal processing part, so that the physical quantity can be determined with accuracy. Furthermore, this configuration does not require the use of the costly reference voltage generating IC that has been used conventionally. In the configuration, a voltage regulator IC for general purpose use, which has been mounted as the voltage regulator for supplying the supply voltage to the signal processing part and the like, also can be used as an IC for generating the reference voltage with respect to the temperature detecting part and the A/D converter, thereby reducing the cost of manufacturing the device.
In the device for determining the physical quantity of the battery pack of the present invention, the physical quantity to be detected is at least one selected from the group consisting of the output voltage of the battery pack, the charge/discharge current of the battery pack, and the temperature in the battery pack.
Furthermore, preferably, the temperature detecting part includes a fixed resistance element and a thermosensitive resistance element, by which the reference supply voltage from the voltage regulator is divided to be output therefrom. According to this configuration, an output from the A/D converter, which corresponds to an environmental temperature in a temperature variation range, can be obtained by using a simple configuration. This can be achieved by, for example, setting the fixed resistance element and the thermosensitive resistance element such as a thermistor element to have equal resistance values at a temperature having a central temperature value in the predetermined temperature range.
Moreover, preferably, the temperature detecting part is provided so as to be adjacent to the voltage regulator. According to this configuration, variations in output voltage of the voltage regulator with respect to variations in environmental temperature can be reflected with accuracy into a physical quantity value that has been determined.
In addition, preferably, in the storage part, the signal levels, each of which is output from the physical quantity detecting part so as to correspond to a reference value of the physical quantity to be detected that has been supplied to the physical quantity detecting part, are stored in advance so as to correspond to the plurality of temperatures. According to this configuration, based on the reference value of the physical quantity to be detected that corresponds to the ambient temperature from the temperature detecting part, in an actual determination of the physical quantity, a signal level from the physical quantity detecting part is corrected in the signal processing part. Thus, the physical quantity can be determined with higher accuracy by correcting temperature characteristics in the physical quantity detecting part.